1. Field of the Invention
The present invention relates to an apparatus for measuring blood constituents, and more particularly to an apparatus for measuring blood constituents by utilizing suction effusion fluid (SEF) obtained by sucking the surface of skin deprived of the keratinous layer.
2. Description of the Prior Art
According to the prior art, when a blood constituent, for instance blood-sugar, is to be measured, it can be done either with blood sampled from a blood vessel or by sampling body fluid containing the blood constituent from under the skin by decompressive suction and measuring the concentration of glucose contained in the SEF with a glucose sensor. Examples of apparatus for measuring blood constituents by utilizing SEF are disclosed in the Japanese Patent Disclosure Gazettes No. 2-31740 published on Feb. 1, 1990 and No. 2-286132 published on Nov. 26, 1990. These apparatuses have the advantage of making it possible to sample body fluid containing the desired blood constituent with no pain to the subject.
FIG. 4 shows a cross section of a blood constituent measuring apparatus embodying the common elements of the aforementioned two examples of the prior art. In the figure, the blood constituent measuring apparatus includes a sampling cell 51 and a measuring section 57 for measuring the concentration of glucose in SEF obtained by suction with the sampling cell 51. The sampling cell 51 consists of a head 52 which comes into contact with skin S, a box 53 solidly linked to the head 52, and a measuring member 54 installed to be rotatable via drive means (not shown) within the box 53. The cylindrical measuring member 54 is rotatable around its center axis (in a direction normal to the surface of the drawing).
Within the head 52 and the measuring member 54 are formed passages 52A and 54A for sucking SEF, which are so arranged as to extend on coaxial lines when decompressive suction is performed.
To the center of the upper face of the box 53 is connected a tube 55 linked to a pump for decompressive suction (not shown), and this tube 55 is so arranged as to be in communication with the passage 54A of the measuring member 54. To the left and right faces of the box 53 are respectively connected a buffer solution feed pipe 56 for supplying buffer solution and an SEF feed pipe 58 for supplying SEF together with the buffer solution to the measuring section 57. The buffer solution feed pipe 56 and the SEF feed pipe 58 are so arranged on the same axis as to permit communication between each other when the passage 54A has reached a substantially horizontal position as a result of the rotation of the measuring member 54 in direction C in the drawing. After the measurement is completed, the SEF is discharged through a pipe 60 together with the buffer solution.
In the above-described configuration, when body fluid containing the desired blood constituent is to be sucked from under the skin, the head 52 of the sampling cell 51 is brought into contact with the surface of skin S, and the passage 54A of the measuring member 54 is so set as to be on the same axis with the passage 52A of the head 52 and with the tube 55.
Then, as the pump for decompressive suction (not shown), connected to one end of the tube 55, is operated to reduce the pressure in the passages 52A and 54A and the tube 55, body fluid containing the desired blood constituent effuses from the surface of skin S into the passages 52A and 54A. When the SEF effusing in this manner has accumulated to a prescribed amount, the measuring member 54 is rotated in direction C to a substantially horizontal position. In this state, the passage 54A in the measuring member 54 is aligned on the same axis with the buffer solution feed pipe 56 and the SEF feed pipe 58; a prescribed amount of buffer solution is supplied to the passage 54A side from the buffer solution feed pipe 56 and, together with the SEF in the passage 54A, supplied to the measuring section 57 of the next stage via the SEF feed pipe 58. During this process, the inside of the measuring section 57 is maintained at normal pressure.
The SEF supplied to the measuring section 57 is subjected by a sensor 59 to prescribed measurement or analysis and, after its completion, discharged externally through the pipe 60 connected to the measuring section 57.
However, since this blood constituent measuring apparatus according to the prior art samples SEF by decompressive suction, the SEF is subject to the presence of bubbles resulting from cutaneous respiration or attributable to oxygen and other gases contained in the SEF itself, and the volume of the gaseous contents of the SEF becomes greater than under normal pressure.
Therefore, in the blood constituent measuring apparatus according to the prior art, which samples SEF under reduced pressure, the volume of bubbles in the passage 54A of the measuring member 54 becomes greater than the volume under normal pressure, however, the bubbles contract when the SEF is supplied to the measuring section 57 under normal pressure, and accordingly the total volume of the SEF obtained eventually becomes much smaller than what it was in the passage 54A of the measuring member 54 and the accuracy of measurement or analysis is thereby affected seriously.